First Android app in Android Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What will this Android app display?

Consider this simple Android app code snippet. What text will appear on the screen when the app runs?

Android Kotlin

import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.material3.Text
import androidx.compose.material3.MaterialTheme
import androidx.compose.material3.Surface
import androidx.compose.runtime.Composable

class MainActivity : ComponentActivity() {
  override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContent {
      Greeting("Android Learner")
    }
  }
}

@Composable
fun Greeting(name: String) {
  Text(text = "Hello, $name!")
}

A Hello, World!

B Hello, Android Learner!

C Greeting Android Learner

D App crashed with error

💡 Hint

Look at the Greeting composable and what string it uses.

The Greeting composable displays the text "Hello, $name!" where name is "Android Learner" passed from setContent. So the screen shows "Hello, Android Learner!".

❓ lifecycle

intermediate

When is onCreate() called in an Android app?

In the Android app lifecycle, when does the onCreate() method run?

A When the app is closed by the system

B Every time the app goes to background

C When the app is first launched and the activity is created

D When the user presses the back button

💡 Hint

Think about the first step when an activity starts.

onCreate() is called once when the activity is first created, before it becomes visible to the user.

📝 Syntax

advanced

What error does this Kotlin code produce?

Look at this Kotlin code snippet for an Android app. What error will it cause?

Android Kotlin

import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.material3.Text

class MainActivity : ComponentActivity() {
  override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContent {
      Text(text = "Welcome")
    }
  }

  fun Text(text: String) {
    println(text)
  }
}

A Compilation error: Conflicting function names for Text

B Runtime error: NullPointerException

C No error, app displays "Welcome" text

D Syntax error: Missing import for Text

💡 Hint

Check if defining a function named Text conflicts with Compose's Text composable.

Defining a function named Text in the same scope conflicts with the Compose Text composable, causing a compilation error due to ambiguity.

❓ navigation

advanced

What happens when this navigation code runs?

Given this Android Jetpack Compose navigation snippet, what screen will the user see after clicking the button?

Android Kotlin

import androidx.compose.material3.Button
import androidx.compose.material3.Text
import androidx.navigation.NavController

@Composable
fun FirstScreen(navController: NavController) {
  Button(onClick = { navController.navigate("second") }) {
    Text("Go to Second")
  }
}

@Composable
fun SecondScreen() {
  Text("Second Screen")
}

A The app navigates to SecondScreen and shows "Second Screen" text

B The app stays on FirstScreen and shows the button

C The app crashes with navigation error

D The app shows a blank screen

💡 Hint

Look at what navController.navigate("second") does.

Calling navController.navigate("second") moves the app to the SecondScreen composable, which displays "Second Screen" text.

🧠 Conceptual

expert

Why use setContent in an Android Compose app?

What is the main purpose of calling setContent in an Android app using Jetpack Compose?

A To start a background service for the app

B To set the app's theme colors and styles globally

C To register event listeners for user input

D To define the UI content of the activity using composable functions

💡 Hint

Think about how Compose replaces XML layouts.

setContent replaces the traditional XML layout by setting the UI content directly with composable functions in Kotlin code.

Practice

1. What is the role of the MainActivity class in a simple Android app?

easy

A. It controls what the user sees by loading the layout and handling interactions.

B. It stores user data permanently on the device.

C. It manages network connections for the app.

D. It compiles the app code into machine language.

Solution

1. Step 1: Understand MainActivity's purpose

   MainActivity is the entry point of the app that sets the screen content.

2. Step 2: Identify its function in UI

   It loads the layout file which defines what the user sees and handles user actions.

3. Final Answer:

   It controls what the user sees by loading the layout and handling interactions. -> Option A

4. Quick Check:

   MainActivity controls UI = A [OK]

Hint: MainActivity sets the screen content and user interaction [OK]

Common Mistakes:

• Confusing MainActivity with data storage classes
• Thinking MainActivity handles network tasks
• Believing MainActivity compiles code

2. Which of the following is the correct way to set the content view in MainActivity using Kotlin?

easy

A. setContentView(R.id.activity_main)

B. setContentView(R.layout.activity_main)

C. setContentView(activity_main.xml)

D. setContentView("activity_main")

Solution

1. Step 1: Recall setContentView syntax

   In Kotlin, setContentView expects a layout resource ID, accessed as R.layout.filename.

2. Step 2: Match correct resource reference

   R.layout.activity_main correctly points to the layout XML file named activity_main.xml.

3. Final Answer:

   setContentView(R.layout.activity_main) -> Option B

4. Quick Check:

   Layout resource ID = R.layout.filename [OK]

Hint: Use R.layout.filename to reference layout files [OK]

Common Mistakes:

• Using string names instead of resource IDs
• Confusing layout and id resources
• Including file extensions in code

3. Given this Kotlin code in MainActivity:

override fun onCreate(savedInstanceState: Bundle?) {
  super.onCreate(savedInstanceState)
  setContentView(R.layout.activity_main)
}

What will the user see when the app runs?

medium

A. A blank white screen with no text

B. An error message because onCreate is missing a return type

C. Nothing, the app will crash immediately

D. The layout defined in activity_main.xml displayed on screen

Solution

1. Step 1: Understand onCreate behavior

   onCreate is called when the app starts and sets the UI with setContentView.

2. Step 2: Effect of setContentView

   It loads the layout file activity_main.xml, so the user sees that layout on screen.

3. Final Answer:

   The layout defined in activity_main.xml displayed on screen -> Option D

4. Quick Check:

   setContentView loads layout = C [OK]

Hint: onCreate sets UI with setContentView(layout) [OK]

Common Mistakes:

• Thinking onCreate needs a return type
• Assuming no UI appears without extra code
• Confusing layout with id resources

4. What is wrong with this MainActivity code snippet?

override fun onCreate(savedInstanceState: Bundle?) {
  setContentView(R.layout.activity_main)
  super.onCreate(savedInstanceState)
}

medium

A. setContentView should be called outside onCreate

B. savedInstanceState should be nullable (Bundle?)

C. super.onCreate(savedInstanceState) must be called before setContentView

D. onCreate must return a value

Solution

1. Step 1: Check onCreate call order

   super.onCreate must be called first to properly initialize the activity.

2. Step 2: Identify correct order

   Calling setContentView before super.onCreate causes errors or unexpected behavior.

3. Final Answer:

   super.onCreate(savedInstanceState) must be called before setContentView -> Option C

4. Quick Check:

   super.onCreate first = B [OK]

Hint: Always call super.onCreate first inside onCreate [OK]

Common Mistakes:

• Calling setContentView before super.onCreate
• Using non-nullable Bundle parameter
• Expecting onCreate to return a value

5. You want your first Android app to show a greeting message centered on a white background. Which layout XML snippet achieves this?

hard

A.

B.

C.

D.

Solution

1. Step 1: Check layout size and background

   uses match_parent for width and height and sets background to white (#FFFFFF).

2. Step 2: Verify text centering

   android:gravity=\"center\" centers the TextView inside the LinearLayout.

3. Final Answer:

   -> Option A

4. Quick Check:

   Center gravity + white background = A [OK]

Hint: Use LinearLayout with gravity=center and background=#FFFFFF [OK]

Common Mistakes:

• Using wrap_content for parent layout size
• Missing gravity=center for centering
• Not setting background color